Electrical elements operable as thermisters, varisters, smoke and moisture detectors, and methods for making the same

ABSTRACT

Electrical elements comprising a metal substrate, a ceramic semiconductor applied thereon and an outer conductive coating. The metal substrate may be a ferrous metal, preferably stainless steel, and may also be a non-ferrous metal, such as aluminum. A bonding agent, such as nickel aluminide is applied to ferrous metal substrates by plasma spraying. The ceramic semiconductor material is also applied to the metal substrate by plasma spraying, and the preferred ceramic semiconductor material for use with a ferrous metal substrate is aluminia titania. The ceramic semiconductor material is coated with a plasma sprayed conductive layer of copper-glass frit to which terminals may be easily soldered. In one configuration, the metal substrate is a long thin rod and the bonding agent, ceramic semiconductor, and copper-glass frit are applied thereto to form cylindrical coatings. Smaller elements are made by cutting the coated rod into short lengths. One terminal is soldered to the copper-glass frit, and the other terminal is the central wire. This configuration displays some polarity. A second configuration comprises grinding away strips of the copper-glass frit wherein two isolated terminals may be attached thereto. The resultant electrical element has the characteristics of a resistance drop in response to either an increase in temperature or humidity, and may be included in a resistance-sensitive alarm circuit. It is desirable to coat electrical elements used as temperature sensors with a water proof coating, so that they do not respond to changes in humidity.

United States Patent [191 Groves et al.

1 ELECTRICAL ELEMENTS OPERABLE AS THERMISTERS, VARISTERS, SMOKE AND MOISTURE DETECTORS, AND METHODS FOR MAKING THE SAME [76] Inventors: Alexander J. Groves, 2625 Park Ave., Bridgeport, Conn. 06604; Alexander J. Groves, Jr., 596 Toilsome Hill, Fairfield, Conn. 06430 [22] Filed: June 15, 1972 [21] Appl. No.: 263,192

[52] US. Cl. ..338/25,117/71 M, l17/93.l PF, 117/105.2, 117/221, ZOO/61.03, 200/6104, ZOO/61.06, 338/21, 338/22,. 338/34, 338/35, 338/308, 340/235, 340/237 S [51] Int. Cl 1101c 7/00, HOlc 13/00 [581 Field of Search 338/20-25, 338/308, 34-35; 117/70 C, 71 M, 217, 215,

227, 93.1 PF, 221, 230, 105.2, 70 C;

[56] References Cited UNITED STATES PATENTS 3,406,366 10/1968 Kontrimas et al. 338/25 3,430,336 3/1969 Riddel 338/22 R 3,650,826 3/1972 Ganser...., 117/217 Primary E.\'aminerC. L. Albritton Attorney, Agent, or FirmMattern, Ware and Davis [57] ABSTRACT Electrical elements comprising a metal substrate, a ce- 1 51 Mar. 18, 1975 ramic semiconductor applied thereon and an outer conductive coating. The metal substrate may be a ferrous metal, preferably stainless steel, and may also be a non-ferrous metal, such as aluminum. A bonding agent, such as nickel aluminide is applied to ferrous metal substrates by plasma spraying. The ceramic semiconductor material is also applied to the metal substrate by plasma spraying, and the preferred ceramic semiconductor material for use with a ferrous metal substrate is aluminia titania. The ceramic semiconductor material is coated with a plasma sprayed conductive layer of copper-glass frit to which terminals may be easily soldered.

In one configuration, the metal substrate is a long thin rod and the bonding agent, ceramic semiconductor, and copper-glass frit' are applied thereto to form cylindrical coatings. Smaller elements are made by cutting the coated rod into short lengths. One terminal is soldered to the copper-glass frit, and the other terminal is the central wire. This configuration displays some polarity. A second configuration comprises grinding away strips of the copper-glass frit wherein two isolated terminals may be attached thereto.

The resultant electrical element has the characteristics of a resistance drop in response to either an increase in temperature or humidity, and may be included in a resistance-sensitive alarm circuit. It is desirable to coat electrical elements used as temperature sensors with a water proof coating, so that they do not respond to changes in humidity.

28 Claims, 11 Drawing Figures 'PMENIEUM R 8195 3,872.4

sum, 1 of 2 BACKGROUND OF THE INVENTION This invention relates to electrical elements, and more particularly to electrical elements useful as thermisters, heat, humidity and moisture detectors, smoke detectors, and varisters. The invention also relates to methods of fabricating electrical elements having the above characteristics.

PRIOR ART Semiconductor materials sandwiched between two electrodes for the purpose of fire detection have been proposed in the prior art. These devices have been designed primarily for detecting fires in aircraft, and are not responsive to small temperature changes in the range immediately above room temperature, wherein early fire detectionin the home could be accomplished.

Further, these devices have generally been expensive to fabricate including sealing the devices from the possible absorption of oxygen, and providing complicated arrangements to insure conductivity between the semiconductor material and the electrodes.

Therefore, there has been a long felt need for an inexpensive, rugged, electrical element useful in detecting temperature changes in the range slightly above room temperature, wherein the electrical element can be included in alarm systems for early detection of tires in homes and other buildings.

There has also been a long felt need for an inexpensive electrical element responsive to moisture and changes in humidity, wherein such elements can be included in a variety of alarm systems for such uses as warning of water in basements, warning of unacceptable humidity levels in storage areas, or monitoring of humidity for medical purposes, and for a variety of other uses wherein it is desirable to monitor relative humidity or detect moisture.

There has also been a long felt need for an electrical element which is responsive to smoke, wherein early detection of fires can be accomplished by sensing the smoke from the fires before any significant increase in room temperature occurs. The electrical elements which are useful in detecting smoke may also be used in the outside environment where large increases in the temperature of the air would not be likely to occur. A potential use for such a device may lie in the field of pollution monitoring.

OBJECTS OF THE INVENTION It is an object of this invention to provide an electrical element which operates as a thermister.

It is another object of this invention to provide an electrical element which is responsive to changes in humidity.

It is an additional object of the invention to provide an electrical element which is responsive to moisture.

It is a further object of this invention to provide an electrical element which is responsive to smoke.

It is another object of this invention to provide electrical elements having the above characteristics which are small.

It is another object of this invention to provide electric elements which are rugged and inexpensive.

LII

It is an additional object of the invention to provide a element which is readily includable in alarm circuits.

It is a still further object of this invention to provide methods for manufacturing such elements.

SUMMARY OF THE INVENTION Electrical elements according to the invention herein comprise a metal substrate having a ceramic semiconductor material applied thereon by plasma spraying. The preferred metal substrate is ferrous metal, and a bonding agent comprising nickel aluminide is applied to the ferrous metal substrate by plasma spraying prior to plasma spraying the ceramic semiconductor thereon. A conductive coating of copper-glass frit is plasma sprayed over the ceramic semiconductor material, and the conductive coating may be separated into at least two portions each comprising one terminal.

The use of plasma spraying techniques fabricating the electrical elements results in properties of extreme strength and ruggedness. The materials are not only joined together by a strong mechanical bond, but also exhibit very good conductivity between the various lay ers despite exposure to moisture, oxygen, and the like. The electrical elements are preferably fabricated in long rods, and the substrate is preferably stainless steel rod having a diameter of 0.070 to .125 inches. After applying the various coatings, the elongated electrical element is cut into short lengths to form a plurality of small electrical elements each approximately 0.75 to 0.8 inches in length. The electrical elements which are to be used as temperature sensors have a portion of the copper-glass frit ground away near the ends of the element to expose a collar whereby electrical arcing at the ends is minimzed. The electrical elements which are used for moisture detectors, humidity detectors and smoke detectors have a greater portion of the copperglass frit gound away, the ground away portion preferably taking the form of two flanking longitudinal strips which also serve to separate the glass copper frit into two distinct terminals.

The invention accordingly comprises the several. steps and the relation of one or more of such steps with respect to each of the others, and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

DRAWINGS For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of an electrical element according to the invention;

FIG. 2 is a side elevation view, partially in section, of the electrical element of FIG. 1;

FIG. 3 is an end view of the electrical element of FIG. 1; w

FIG. 4 is a perspective view of the electrical element of FIG. 1 having two longitudinal portions of the surface coatingground away;

FIG. 5 is an end view of the electrical element of FIG. 4.

FIG. 6 is an end view of an electrical element similar to FIG. 1;

FIG. 7 is a schematic perspective view of apparatus for manufacturing the electrical element of FIG. 1;

FIG. 8 is a graph of resistance vs. temperature for the electrical element of FIG. 1;

FIG. 9 is a graph of resistance vs. temperature for the configuration of the electrical element of FIG. 4;

FIG. 10 is a graph of resistance vs. relative humidity for the electrical element of FIG. 4;

FIG. 11 is a graph of resistance vs. voltage for the the electrical element of FIG. 1.

The same reference numbers refer to the same elements throughout the various drawings.

PREFERRED EMBODIMENT The electrical elements disclosed herein are sensitive to heat, smoke, humidity, moisture, and voltage. Thus, they are very useful as sensors in alarm systems for home and industrial environmentsfTherefore, although the electrical elements and the methods for making the same may be adapted for other uses, the preferred embodiments discussed below relate to electrical elements 7 and the methods of fabricating the same wherein such elements are useful as sensors in home and industrial alarm systems.

The electricalelements generally comprise a metal substrate having a ceramic semiconductor applied thereon, and having a conductor deposited on the surface of the ceramic semiconductor for use as a terminal. The method of manufacturing such elements generally comprises plasma spraying the ceramic semiconductor material onto the metal substrate, and plasma spraying the conductive material onto the ceramic semiconductor. A long electrical element is fabricated, and the long electrical element is thereafter cut into smaller electrical elements which are appropriately fin ished by grinding away portions of the sprayed materials, and attaching lead wires.

The metal substrate may be any ferrous metal or some nonferrous metals, such as aluminum. The preferred metal substrate is amild stainless steel rod having a diameter in the range of0.075 0.125 inches, and preferably 0.090 inches. It is convenient to work with a stainless steel rod several feet in length.

The rod is prepared for plasma spraying by abrading the surface thereof with 40 mesh aluminide oxide, commercially available as Metcolite S from Metco, Inc., Westbury, Long Island, New York. A relatively fine mesh abrading material is used as it is desirable to merely remove contaminates from the surface of the rod and slightly toughen the surface of the rod without pitting or removing any substantial portion thereof.

Referring now to FIG. 7, a stainless steel substrate rod 10 is placed in chunks 11 and 12 of a turning machine 13 for rotation therein. One of the chucks is preferably spring loaded wherein expansion of the rod 10 due to heating may be accommodated. The turning machine 13 is provided with a threaded drive rod 14 and associated fixture 16' for transversing a plasma spray gun 17 along the length of the rod 10. A transverse speed of 25 feet per minute is satisfactory. A suitable plasma spray gun is Metcos'model M3B, and details of plasma spray techniques are described in the Metco Flame Spray Handbook, Volume III Plasma Flame Process, published by Metco, Inc. The preferred plasma for use herein is a mixture of 90 percent Nitrogen (N2) and 10 percent Hydrogen (H and the gun is operated at temperatures in the range of l0,000 F to 15,000 F, typically 12,000 F.

The plasma spray gun 17 is used to first preheat the stainless steel rod 10 to approximately 250 F.

Ceramic semiconductor materials can be plasma sprayed directly onto the stainless steel or ferrous metal substrates; however, depending upon the choice of materials, the strength of the bond between the ceramic semiconductor and the metal substrate can be enhanced by use of a bonding agent.

The preferred bonding agent is nickel aluminide, commercially available from Metco. Inc. and identified as its product M-450. The bonding agent is a mixture of'95 percent nickel and 5 percent aluminum, which is applied to the substrate by the plasma spray gun. When subjected to the high temeratures of the plasma spray gun, the mixture of nickel and aluminum undergoes a synergistic exothermic reaction wherein the nickel and aluminum combine into MM and NlgAl. The thickness of the layer of bonding agent applied to the rod 10 may be in the range of 0.001 to 0.010 inches, preferably 0.003 to 0.005 inches.

The preferred semiconductor material for fabricating the electrical elements described herein is alumina titania, which comprises a mixture by weight of 87 percent A1 03 and 13 percent TiO and is avaiable from Metco, Inc. as its product Metco 130. It is advantageous to use the nickel aluminide bonding agent between the alumina titania semiconductor material and the stainless steel substrate, or with any other ferrous metal base.

Other typical semiconductor materials which may be used in fabricating electrical elements of the type described herein are a mixture of alumina and barium oxide (A1 0 BaO), barium titanate (BaTiO calcium titanate (CaOTiO and the like.

The ceramic semiconductor material is plasma sprayed over the bonding agent. As described above, the plasma used is a mixture of 90 percent nitrogen (N and 10 percent hydrogen (H The hydrogen, which disassociates into two single hydrogen atoms at the temperatures achieved in the plasma spray gun, combines with free oxygen present in or around the surface being coated, and therefore wipes away" the oxygen which might otherwise create unwanted oxides.

The thickness of the layer of ceramic semiconductor material may be in the range of 0.003 to 0.015 inches, and is preferably 0.007 inches.

Application of aluminum titania onto metal substrates by means of sintering or baking in furnaces results in a low-porosity coating which is whitish or pale gray in color. Plasma spraying the alumina titania onto the cylindrical stainless steel substrate coated with the bonding agent results in a higher porosity coating which is very dark gray in color. The high porosity of the coating is probably an effect of both the plasma spraying process and the smaller diameter of the surface being coated. In the'plasma spray process, the alumina titania is introduced into a high velocity, high temperature plasma stream wherein the alumina titania is melted into droplets of varying sizes. The larger droplets hit the surface being coated and cool very rapidly, bonding onto neighboring droplets previously deposited and other droplets being simultaneously deposited. The smaller droplets of alumina titania in the gaseous stream may be swept away by the plasma rather than be deposited on the surface. In addition, spraying material onto a small diameter rod always results in some shadow spraying, particularly at the portions of the rods which are tangential to the spray stream. The previously deposited particles cast shadows leaving areas on which material is not deposited. The rotation of the rod during spraying achieves a uniform coating, but the coating is nevertheless more porous because of the shadow effect. 7

The cause of the dark coloring of the ceramic semiconductor material is not known; however, it may be the result of some chemical reaction at the high temperatures achieved in the spray process, or may be some doping effect from the metal substrate or the nickel aluminide bonding agent.

A conductive layer is applied over the ceramic semiconductor. This conductive layer preferably comprises a copper-glass frit commercially available from Metco as XP 1159, the ratio of copper to glass being 3 to l. The copper and glass are ground into a very fine powder and applied by plasma spraying using the nitrogenhydrogen plasma at approximately l2,00 F. It is be lieved that the copper-glass frit is deposted as small glass beads partially coated with copper.

The copper-glass frit is applied to a depth in the range of 0.003 0.015 inches, and preferably 0.007 inches. However, almost any depth of copper-glass frit or any other conductor would suffice as a conductive layer.

The coated rod 10 is removed from the turning machine 13 after the plasma spraying process is completed. Thereafter, individual electrical elements are fabricated from the coated rod 10.

Referring now to FIGS. 1-3, there is shown an electrical element comprising a portion of the coated rod 10. The length of element 20 may be as desired, but it has been found that segments of the rod having length L in the range of 0.5 to 1 inch in length are useful. In FIG. 2, element 20 has length L equal to 0.8 inch.

Element 20 is comprised of the mild stainless steel metal substrate rod 10 havingdiameter D equal to approximately 0.090 inches. The rod has been coated with a first layer 21 of nickel aluminide bonding agent,-

which layer has the thickness B equal to 0.004 inches. The next layer22 of the electrical element 20 comprises the alumina titania ceramic semiconductor. Layer 22 has thickness E, which is preferably 0.007 inches. The outer most layer 23 is the copper-glass frit which has the dimension F equal to 0.007 inches. The various layers were applied by plasma spraying process, as described above.

Referring now to FIGS. 1 and 2, a portion of the copper frit 23 is ground to expose a collar 26 of the semiconductor layer 22. The collar 26 is desirable'to insure that no shorts exist between the copper-glass frit 23 and the rod 10 or bonding agent layer 21. Such shorts are sometimes introduced by smearing of the metal across the semicondcutor layer during the cutting operation. The collar may be very narrow, having the dimension indicated at C of typicallly 0.010 inches.

The electrical element 20 is useful as a fire detector having as an operating principle a decrease in resistance in response to an increase in heat. Accordingly, the electrical element 20 is connected in an alarm circuit 30 by solding a first lead 31 to the copper frit layer 23 at 32, and connecting a second lead 33 to the rod 10 at 34.

The alarm circuit 30 is preferably of the type which provides a DC voltage across the element 20. The leads 31 and 33 may be relatively long, and the element is placed in a position where air heated by a tire would first accumulate, such as near a ceiling, near a furnace, or the like. As the electrical element 20 is also sensitive to humidity or moisture, it may be desirable to clip the element in varnish or to otherwise provide a waterproof coating so that the element will not cause false fire alarms in response to high humidity or moisture. A waterproof coating is particularly desirable when the element is installed in a kitchen where there may be steam from cooking, or in other moist areas.

Referring now to the graph of HO. 8, there are shown two curves A and B of resistance vs. temperature. Curve A is the resistance vs. temperature relationship for element 20 having 50 volts DC potential applied between the leads 31 and 33, lead 31 being negative. At F, a relatively high room temperature, the element 20 has a resistance of 1.9 megaohms; at F, the resistance of element 20 had decreased to less than 1.2 megaohms; and at 130 F, the resistance of element 20 was 0.65 megaohms. Thus, the resistance at 130 F was approximately one-third of the resistance at 80 F, and the total resistance drop was approximately 1.3 megaohms. This large drop in resistance is more than adequate to trigger even a relatively insensitive resistance responsive alarm circuit.

Curve B was derived by reversing the polarity of the two leads 31 and 33, the negative lead being connected to the central rod 10. At 80 F the resistance of element 20 was 1.3 megaohms, and the resistance declined substantially linearally to 0.54 megaohms at F. Curve B therefore also represents a substantial drop in resistance upon which an alarm circuit can be triggered.

The polarity, i.e., the difference between curve A and curve B of FIG. 8, indicates that the element 20 has some of the characteristics of PN junction semiconductor. The conductive metal substrate and the nickel aluminide bonding coat have available free electrons, and is roughly equal to an N type material. It is a conductor rather than a semiconductor. Also, on application of the ceramic material by plasma spraying, the innermost layer adjacent to the metal substrate or the layer of bonding agent may become greatly contaminated with metal, and therefore form a thin layer of N-type ceramic material which would most likely be a semiconductor, although isolation of the material of this layer has not been accomplished and has therefore not been tested.

Referring now to FIG. 6, an alternative electrical element 20C is shown. Element 20C comprises an aluminum substrate in the form of a rod 40, and has a layer 42 of the alumina titania applied thereon by plasma spraying, and a layer 43 of copper-glass frit applied on the ceramic also by plasma spraying. No bonding agent was required in fabricating element 20C. Element 20C displays polarity characteristicssimilar to those of element 20a. Again, the portion of the ceramic material immediately adjacent to the aluminum substrate may become contaminated with aluminum molecules, and thereby comprise a thin layer of N type semiconductor material.

The combination of metal oxides plasma sprayed onto the metal substrate probably take the form of an amorphous conglomeration which is partly crystalline, but irregular. The fact that this material conducts indicates that some modification of the metal oxides is formed, the A1 by itself being an excellent insulator. It is therefore believed that the ceramic material is an irregular crystalline structure wherein some of the aluminum or titanium atoms loosely share electrons in their outer most rings, and wherein some deficiency of electrons exists so that the ceramic material has the characteristics of P-type semiconductor.

This analogy to a pure PN junction semiconductor material would account for the variation in resistance between curve A and curve Bof FIG. 8. Curve A, wherein the central terminal is positive, represents a reverse conduction voltage, wherein higher resistance results. Curve B, wherein the negative terminal is appliedto the metal substrate, is the equivalent of the forward conduction voltage situation, and lower resistance and higher conductivity result.

The decrease in resistance in response to a rise in temperature is most likely caused by thermal excitation of the material, wherein a greater number of electrons are free for conduction. This is a well known phenomena .in semiconductor art. It is a surprising discovery that this phenomena is also found in the devices disclosed herein, which are fabricated so quickly and inexpensively, and that the phenomena occurs at temperatures in the proper range for use as a fire or heat detector in home or industrial environments.

Referring now to FIGS. 4 and 5, there is shown an alternative electrical element 20a. This element is fabricated similarly to the electrical element 20, with the additional step of grinding away two strips of the copperglass frit wherein the copper-glass frit layer 23 is divided into two portions 23a and 23b, and two strips 27 of the semiconductor layer 22 are exposed. A silicon carbide grinding wheel comprising carbon'embedded in compressed paper is used for the grinding operation, as it has sufficient hardness to remove the copper-glass frit but insufficient hardness to remove any of the ceramic semiconductor immediately thereunder. In addition, the wheel is sufficiently flexible to conform to the cylindrical'surface of the ceramic semiconductor layer so that the copper-glass frit is removed over the entire width of the wheel rather than at a line tangential to the wheel. The .preferred width W of the path is 0.04 inches, although the width is by no means critical.

Element 20a is connected to an alarm circuit 30a by means of lead 36 attached to the portion of the copperglass frit 23a at 37, and lead 38 attached to the second portion of the copper-glass frit 23b at 39. The element 200 is preferably dipped in a waterproofing solution when the element is to be used as a-heat sensor. Varnish works well for this purpose.

in the sketch in FIG. 10, i.e.,.one lead is soldered to portion 23a of the copper-glass frit and the second lead is soldered to the rod 10. A low voltage of 1 volt DC is provided across the element by the alarm circuit. At a relative humidity of 40%,the element displayed a resistance of megaohms. The resistance dropped most rapidly between 60% and relative humidity, the resistance having a value of approximately 20 megaohms at 70% relative humidity. A more gradual drop in resistance was displayed between 70 and relative humidity, and at 90% relataive humidity the element displayed a resistance of approximately 5 megaohms. In the range slightly above 90% relative humidity, a sharp drop to about Zmegaohms resulted, although this departure from a smooth curve may have been the result of instrument error. At relative humidity, the resistance of the element had dropped to approximately 1 megaohm, and at 99% relative humidity the resistance was near zero.

The above graph points out the usefulness of the element as a humidity sensor. In combination with a properly designed alarm circuit, the element is sufficiently sensitive to be used as a breath detector for medical applications. The element can also be used for applications wherein monitoring of dry. storage conditions is desired, or the like.

The element also operates as an almost indestructible water level sensor; i.e., dipping the element in water causes a nearly complete loss of resistance. Repeated the wetting and drying of the element results in no loss of its other sensing properties. I

The element is also sensitive to smoke, having the response of decreased resistance. No standard for concentration of smoke was derived, and hence no graph is included herein. However, in tests the element was sufficiently sensitive to trigger an alarm circuit in response to a puff of concentrated cigarette smoke applied directly to the element. It is believed that the smoke sensing characteristic of the element is an extension of the moisture sensing properties of the element, in the smoke contains a substantial amount of water, vapor, acids, and the like.

It is believed that the moisture responsive characteristics of the element are caused by an increased concentration of moisture in the porous ceramic material. I

This increase in concentration may take the form of water vapor, condensed water droplets, or an actual infusion of water, and may also be acids, other electro- Iites, or the like, in similar forms. It is possible that these molecules, loosely embraced in the physical space of the porous ceramic material, share some electrons with the crystal instruction of the ceramic mate- Referring now to FIG. 9, there is shown a graph of re- I sistance vs. temperature for element 23a. The alarm circuit 30a provides a 100 volt DC potential across the element, as indicated in the sketch included with FIG. 9. At 80 F, the element 20a has a resistance of 12 megaohms. The resistance declines in a substantially lin ear manner to approximately 7 megaohms at 128 F. Thus, the element 23a displays a 50 percent loss of resistance in the temperature range immediately above average room temperatures, and is very useful in detecting fires in the home environment.

The element 20a= may also be used to detect moisture, humidity, or smoke. Referring now to FIG. 10, a graph of resistance vs. relative humidity is shown. The element 20a is connected to an alarm circuit as shown rial, and thereby enhance the conductive capabilities thereof. It is also possible that the moisture is sufficiently contaminated to be an electrolite in and of itself, or that it combines with some of the ceramic semiconductor material to become an electrolite and increase conduction.

In any event, the moisture is not believed to penetrate the ceramic material to a very great depth, and consequently the decreased resistivity due to moisture is primarily a surface effect. Therefore the configuration of the element shown in FIG. 4 is preferred for moisture sensing.

The electrical elements 20 and 20A also operate as varisters. Referring now to FIG. 11, there is shown a graph of resistance vs. supplied DC voltage for element 20 at room temperature. The graph of FIG. 11 includes two curves A and B, curve A being derived from applying voltage with the positive terminal connected to the core, and curve B being derived by connecting the negative terminal to the core. The resistance of element 20 with the positive terminal connected to the central core was approximately 3.9 megaohms at 20 volts, and decreased rapidly to about 2.5 megaohms at 40 volts. Thereafter. a more gradual decrease in resistance was observed, the resistance having a value of approxi mately 1.75 megaohms at 70 volts, and about 1.3 megaohms at 100 volts. Curve B, which was derived by connecting the negative terminal of the voltage supply to the center of element 20, resulted in a resistance of 2.4 megaohms at 20 volts, with the resistance decreasing relatively linearly to a value of approximately l megaohm at 70 volts. Thereafter, the resistance remained relatively constant to 100 volts.

Element 20A may therefore be used as a varister, i.e., the resistance of the element is dependenton the applied voltage. The ceramic material apparently operates as a leaky di-electric, and the polarity is again evident. Therefore, the ceramic material, in addition to acting like a leaky di-electric, retains some of the characteristics of a PN junction diode.

The electrical elements disclosed herein may be used as sensors in a variety of alarm systems having many applications. The size of the elements may be varied wherein quantitative differences in the response of the elements can be realized; e.g. a longer element having the same thicknesses of layers of material has a higher initial resistance. Similarly, the elements can be used in alarm circuits providing AC across the terminals, wherein the elements behave somewhat differently but operate along the same principles disclosed herein.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description. are efficiently attained and, since certain changcs may be made in carrying out the above method and in the article set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described our invention, that we claim as new and desire to secure by Letters Patent is:

1. An electrical element comprising:

A) a metal substrate;

B) a layer ofnickel aluminide deposited on the metal substrate;

C) a layer of ceramic semiconductor material comprising aluminum oxide and titanium oxide deposited on the layer of nickel aluminide and D) a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.

2. An electrical element as defined in claim 1 wherein the metal substrate is a ferrous metal.

3. An electrical element as defined in claim 1 wherein the metal substrate is an alloy of iron.

4. An electrical element as defined in claim 1 wherein the metal substrate is stainless steel.

5. An electrical element comprising: A. a stainless steel substrate; B. a layer of nickeland aluminum applied to the stainless steel substrate by plasma spraying at tem- 5 peratures in excess of l0,000 F, wherein the nickel and aluminum combine by means of a synergistic exothermic chemical reaction to form MM and Nia C. a layer of ceramic semiconductor material applied to the layer of nickel and aluminum comprising 75-95 percent aluminum oxide and 5-25 percent titanium oxide; and

D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.

6. An electrical element comprising: A. a metal substrate;

B. a layer of a metallic bonding agent deposited on the metal substrate;-

C. a layer of ceramic semiconductor material deposited on the layer of metallic bonding agent; and

D. A layer of conductive material forming an electrical terminal comprising a metal-glass frit deposited on the layer of ceramic semiconductor material.

7. An electrical element .as defined in claim 6 wherein the metal-glass frit is copper-glass frit.

8. An electrical element as defined in claim 1 wherein the metal substrate is a ferrous metal, the bonding agent is nickel aluminide,- and the layer of conductive material comprises a metal-glass frit.

9. An electrical element as defined in claim 1 wherein the metal-glass frit is copper-glass frit.

10. An electrical element as defined in claim 1 wherein the metal substrate is cylindrical.

11. An electrical element comprising:

A. a cylindrical metal substrate having a diameter in the range of 0.075 inches to 0.125 inches;

B. a layer of metallic bonding agent deposited on the metal substrate;

C. a layer of ceramic semiconductor material deposited on 'the bonding agent; and

v D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.

12. An electrical element as defined in claim 11 wherein the layer of ceramic semiconductor material has a thickness of 0.003 inches to 0.015 inches.

13. An electrical element comprising:

A. a cylindrical ferrous metal substrate;

B. a layer of nickel aluminide bonding agent depos ited on the metal substrate;

C. a layer of ceramic semiconductor material deposited on the layer of bonding agent; and

D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.

14. An electrical element as defined in claim 13 wherein the cylindrical metal substrate has a diameter in the range of 0.075 inches to 0.125 inches.

15. An electrical element as defined in claim 14 wherein the layer of nickel aluminide bonding agent has a depth in the range of 0.001 inches to 0.010 inches.

16. An electrical element as defined in claim wherein the cylindrical metal substrate has a diameter in the range of 0.075 inches to 0.125 inches.

17. An electrical element as defined in claim 16 wherein the layer of aluminum oxide and titanium oxide ceramic semiconductor material has a depth in the range of 0.003 inches to 0.015 inches.

18. An electrical element as defined in claim 17 wherein the layer of nickel aluminide bonding agent has a depth in the range of 0.001 to 0.010 inches.

19. An electrical element comprising:

A. a cylindrical stainless steel substrate having a diameter in the range of 0.075 inches to 0.125 inches;

B. a mixture of nickel and aluminum applied to the stainless steel metal substrate by plasmaspraying temperatures in excess of l0,000 F. wherein the nickel and aluminum combine by means ofa synergistic exothermic reaction to form a layer-of bonding-agent comprising MA] and Ni Al, said layer having a depth of 0.003 to 0.005 inches;

C. a layer of ceramic semiconductor material comprising a mixture of 75-95% aluminum oxide and -25% titanium oxide applied to'the layer of bondin g agent by plasma spraying at temperatures in excess of 10,000 F, wherein the aluminum oxide and titanium oxide form a porous semiconductor material, said layer having a depth of 0.005-0.009 inches; and

D. a layer of conductive material comprising copperglass frit applied over the ceramic semiconductor material by pasma spraying, said layer having a depth of 0.003 to 0.015 inches.

20. An electrical element as defined in claim 19 wherein the length of the cylindrical metal substrate is between 0.5 and 1 inches, and wherein the bonding agent, ceramic semiconductor material, and conductive material are applied over the cylindrical surface of the metal substrate.

21. An electrical element as defined in claim 20 wherein the layer of conductive material is ground away at the ends thereof to expose two collars of the ceramic semiconductor material.

22. An electrical element as defined in claim 21 wherein the collars have a width of approximately 0.01 inches.

23. An electrical element as defined in claim 19 wherein two flanking longitudinal strips of the copperglass frit are ground away to expose two strips of the ceramic semiconductor material and to separate the copper-glass frit into two portions. I

24. An electrical element as defined in claim 23 wherein two flanking longitudinal strips of the copperglass frit are ground away to expose two strips of the ceramic semiconductor material and to separate the copper-glass frit into two portions.

25. An electrical element as defined in claim 19 wherein a first electrical lead is connected to the stainless steel metal substrate and wherein a second electrical lead is connected to the copper-glass frit.

26. An electrical element as defined in claim 25 wherein the electrical element is covered with a water proof coating.

27. An electrical element as defined in claim 23 wherein a first electrical lead is connected to one portion of the copper-glass frit and a second electrical lead is connected to the second portion of the copper-glass frit.

28. An electrical element comprising a body of ceramic semiconductor material comprising a mixture of 95% aluminum oxide and 5-25% titanium oxide formed by plasma spraying at temperatures in excess of l0,000 F., wherein the aluminum oxide and titanium xide form a cohesive porous semiconductor material and at least two separate metallic electrodes each in electrical contact with a different area of the surface of said body. 

1. AN ELECTRICAL ELEMENT COMPRISING: A) A METAL SUBSTRATE; B) A LAYER OF NICKEL ALUMINIDE DEPOSITED ON THE METAL SUBSTRATE; C) A LAYER OF CERAMIC SEMICONDUCTOR MATERIAL COMPRISING ALUMINUM OXIDE AND TITANIUM OXIDE DEPOSITED ON THE LAYER OF NICKEL ALUMINIDE AND D) A LAYER OF CONDUCTIVE MATERIAL FORMING AN ELECTRICAL
 2. An electrical element as defined in claim 1 wherein the metal substrate is a ferrous metal.
 3. An electrical element as defined in claim 1 wherein the metal substrate is an alloy of iron.
 4. An electrical element as defined in claim 1 wherein the metal substrate is stainless steel.
 5. An electrical element comprising: A. a stainless steel substrate; B. a layer of nickel and aluminum applied to the stainless steel substrate by plasma spraying at temperatures in excess of 10, 000* F, wherein the nickel and aluminum combine by means of a synergistic exothermic chemical reaction to form NiAl and Ni3Al; C. a layer of ceramic semiconductor material applied to the layer of nickel and aluminum comprising 75-95 percent aluminum oxide and 5-25 percent titanium oxide; and D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.
 6. An electrical element comprising: ''A. a metal substrate; B. a layer of a metallic bonding agent deposited on the metal substrate; C. a layer of ceramic semiconductor material deposited on the layer of metallic bonding agent; and D. A layer of conductive material forming an electrical terminal comprising a metal-glass frit deposited on the layer of ceramic semiconductor material.
 7. An electrical element as defined in claim 6 wherein the metal-glass frit is copper-glass frit.
 8. An electrical element as defined in claim 1 wherein the metal substrate is a ferrous metal, the bonding agent is nickel aluminide, and the layer of conductive material comprises a metal-glass frit.
 9. An electrical element as defined in claim 1 wherein the metal-glass frit is copper-glass frit.
 10. An electrical element as defined in claim 1 wherein the metal substrate is cylindrical.
 11. An electrical element comprising: A. a cylindrical metal substrate having a diameter in the range of 0.075 inches to 0.125 inches; B. a layer of metallic bonding agent deposited on the metal substrate; C. a layer of ceramic semiconductor material deposited on the bonding agent; and D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.
 12. An electrical element as defined in claim 11 wherein the layer of ceramic semiconductor material has a thickness of 0.003 inches to 0.015 inches.
 13. An electrical element comprising: A. a cylindrical ferrous metal substrate; B. a layer of nickel aluminide bonding agent deposited on the metal substrate; C. a layer of ceramic semiconductor material deposited on the layer of boNding agent; and D. a layer of conductive material forming an electrical terminal deposited on the layer of ceramic semiconductor material.
 14. An electrical element as defined in claim 13 wherein the cylindrical metal substrate has a diameter in the range of 0.075 inches to 0.125 inches.
 15. An electrical element as defined in claim 14 wherein the layer of nickel aluminide bonding agent has a depth in the range of 0.001 inches to 0.010 inches.
 16. An electrical element as defined in claim 10 wherein the cylindrical metal substrate has a diameter in the range of 0.075 inches to 0.125 inches.
 17. An electrical element as defined in claim 16 wherein the layer of aluminum oxide and titanium oxide ceramic semiconductor material has a depth in the range of 0.003 inches to 0.015 inches.
 18. An electrical element as defined in claim 17 wherein the layer of nickel aluminide bonding agent has a depth in the range of 0.001 to 0.010 inches.
 19. An electrical element comprising: A. a cylindrical stainless steel substrate having a diameter in the range of 0.075 inches to 0.125 inches; B. a mixture of nickel and aluminum applied to the stainless steel metal substrate by plasma spraying temperatures in excess of 10,000* F. wherein the nickel and aluminum combine by means of a synergistic exothermic reaction to form a layer of bonding agent comprising NiAl and Ni3Al, said layer having a depth of 0.003 to 0.005 inches; C. a layer of ceramic semiconductor material comprising a mixture of 75-95% aluminum oxide and 5-25% titanium oxide applied to the layer of bonding agent by plasma spraying at temperatures in excess of 10,000* F, wherein the aluminum oxide and titanium oxide form a porous semiconductor material, said layer having a depth of 0.005-0.009 inches; and D. a layer of conductive material comprising copper-glass frit applied over the ceramic semiconductor material by pasma spraying, said layer having a depth of 0.003 to 0.015 inches.
 20. An electrical element as defined in claim 19 wherein the length of the cylindrical metal substrate is between 0.5 and 1 inches, and wherein the bonding agent, ceramic semiconductor material, and conductive material are applied over the cylindrical surface of the metal substrate.
 21. An electrical element as defined in claim 20 wherein the layer of conductive material is ground away at the ends thereof to expose two collars of the ceramic semiconductor material.
 22. An electrical element as defined in claim 21 wherein the collars have a width of approximately 0.01 inches.
 23. An electrical element as defined in claim 19 wherein two flanking longitudinal strips of the copper-glass frit are ground away to expose two strips of the ceramic semiconductor material and to separate the copper-glass frit into two portions.
 24. An electrical element as defined in claim 23 wherein two flanking longitudinal strips of the copper-glass frit are ground away to expose two strips of the ceramic semiconductor material and to separate the copper-glass frit into two portions.
 25. An electrical element as defined in claim 19 wherein a first electrical lead is connected to the stainless steel metal substrate and wherein a second electrical lead is connected to the copper-glass frit.
 26. An electrical element as defined in claim 25 wherein the electrical element is covered with a water proof coating.
 27. An electrical element as defined in claim 23 wherein a first electrical lead is connected to one portion of the copper-glass frit and a second electrical lead is connected to the second portion of the copper-glass frit.
 28. An electrical element comprising a body of ceramic semiconductor material comprising a mixture of 75-95% aluminum oxide and 5-25% titanium oxide formed by plasma spraying aT temperatures in excess of 10,000* F., wherein the aluminum oxide and titanium oxide form a cohesive porous semiconductor material and at least two separate metallic electrodes each in electrical contact with a different area of the surface of said body. 